P-aminodiphenylamines are widely used as intermediates in the manufacture of alkylated derivatives, which find broad utility as antiozonants and antioxidants. For example, alkylation of p-aminodiphenylamine (PADPA) with acetone provides the broadly used antiozonant N-phenyl-N'-isopropyl-p-phenylenediamine.
The most accessible route to p-aminodiphenylamines involves N-nitrosation of a diphenylamine, rearrangement of the resulting N-nitrosodiphenylamine (NNODPA) to the corresponding C-nitrosodiphenylamine, e.g., p-nitrosodiphenylamine (PNODPA), and subsequent hydrogenation of the latter to the p-aminodiphenylamine. It long has been known that the procedure is fraught with difficulties, some of which result from objectionable properties of p-nitrosodiphenylamine, which is representative of the class of C-nitrosodiphenylamines. In particular, the latter are carcinogens requiring extensive precautions in their handling, a problem compounded by the fact that p-nitrosodiphenylamine is often obtained as a light powder, easily dispersed and airborne. Another difficulty arises from the thermal instability of p-nitrosodiphenylamines, so that there is a tendency during their subsequent hydrogenation to form highly-colored byproducts which are difficult to remove from PADPA and which substantially reduce the commercial value of the latter material.
U.S. Pat. No. 2,974,169 describes the reduction of an aqueous solution of an alkali metal salt of PNODPA with palladium on charcoal. This method has the advantage of obviating, in large part, the formation of objectionable color bodies. However, catalytic reduction in aqueous media presents substantial disadvantages, and because the patentees contemplate isolation of PNODPA the necessity of handling carcinogens remains. These problems were addressed and overcome in U.S. Pat. No. 4,313,002, where the patentees' discovery of the solubility in particular organic solvents of some metal salts of p-nitrosodiphenylamines made possible the catalytic reduction of the latter in non-aqueous media without the necessity of isolating and handling the p-nitrosodiphenylamine.